My brass isn't quite as red as theirs is, but with a wash of Agrax Earthshade the shine is dulled sufficiently:
Pre-wash Juggerspawn 1:
Post-wash Juggerspawn 2:
I'm debating the use of Reikland Fleshshade to add an even redder tint to the brass, but this might be good enough.
Also, I'm painting the chainmail skirts silver. Wasn't even thinking as I painted them brass. That's just goofy! Really it adds a little bit of much-needed contrast to an otherwise two color model.